Dense Breast Legislation and Supplemental Breast Imaging Among Women Undergoing Mammography.

Importance: State-level dense breast legislation aims to increase awareness of breast density and promote supplemental imaging, yet the association of specific legislation type with imaging use remains unclear. Objectives: To evaluate the differential association of dense breast notification and insurance coverage laws with supplemental imaging use and to assess how specific legislative features are associated with imaging use. Design, Setting, and Participants: This retrospective, quasi-experimental cohort study used data from the MarketScan database, one of the largest national administrative claims databases in the US, from January 1, 2008, through December 31, 2019. A total of 12.6 million women aged 40 to 64 years undergoing screening mammography with continuous insurance enrollment for at least 6 months after the index screening mammography were followed up for up to 6 months after the screening mammography. Data were analyzed from January to July 2025. Intervention: Dense breast legislation, including notification and insurance coverage laws. Main Outcomes and Measures: Receipt of supplemental ultrasonography or magnetic resonance imaging (MRI) within 6 months of screening mammography. Multivariable logistic regression with an extended difference-in-differences design was used to assess within-state changes over time, leveraging variation in the timing of policy adoption across states. Results: Of 12 555 082 women (mean [SD] age, 51.3 [6.9] years) undergoing mammography, 9.0% received supplemental ultrasonography and 0.6% received supplemental MRI. Notification mandates were associated with higher use of ultrasonography (odds ratio [OR], 1.09; 95% CI, 1.08-1.10) and MRI (OR, 1.09; 95% CI, 1.06-1.12). Insurance coverage was associated with higher odds of supplemental ultrasonography use (OR, 1.88; 95% CI, 1.86-1.90). Among states with notification mandates, including information on supplemental screening (OR, 2.84; 95% CI, 2.81-2.86) and targeting notification only to women with dense breasts (OR, 1.32; 95% CI, 1.31-1.33) were both associated with higher odds of ultrasonography use. Conclusions and Relevance: In this cohort study of women undergoing mammography, both dense breast notification and insurance coverage laws were associated with increased use of supplemental ultrasonography, with greater odds observed for insurance mandates. Specific legislative features, particularly targeted notification and inclusion of supplemental screening information, were also associated with increased use of supplemental ultrasonography. Further studies covering wider populations and integrating claims data with clinical registries, imaging reports, or electronic health records are needed to better delineate the association between dense breast legislation and supplemental screening behavior.